Re: Begining training in Aikido
Personally just from my own experience I don't think you need to do anything in particular to prepare. They certainly shouldn't expect you to know anything at all on your first day - that's the point of a school, and they'll have their own preferred methods of teaching anyway. And most things are safer to learn with instruction.
If you're not in good enough basic shape (cardio, flexibility, old injuries, etc) to get your heart rate up and to get into weird positions without risking hurting yourself, then I'd address that... but I don't know that you need to be in great shape already just to start a beginner's class. It's for beginners, that's the point... I think you'll figure out after you start doing it what you're lacking and if there's anything extra that would help. As long as you pace yourself and respect your limitations you will adapt.
The kind of stuff I do personally that I find helps me most in aikido (besides doing aikido) is body weight stuff like planks and squats and standing on one leg (i.e., core, legs, balance, range of motion) and things that use cardio. But (other than cycling) I didn't start doing any of that stuff until long after I started aikido. And different people have different strengths and weaknesses (e.g. I'm pretty flexible so that's never been something I had to consciously think about).
You'll kind of get a better idea once you're actually in the thick of it.